The present invention relates to horizontal deflection circuits for use in a television set or video display unit. In particular, it relates to high-frequency horizontal deflection circuits for obtaining high picture quality.
Generally, a conventional horizontal deflection circuit of a television set or video display unit supplies sawtooth current at a repetitive frequency of 15.75 kHz to a horizontal deflection coil. FIG. 8 shows the fundamental horizontal deflection circuit in which a horizontal deflection coil 1, a tuning capacitor 2, a damper diode 4, and a switching element 3 are provided, a bipolar transistor normally being used as the switching element 3. FIG. 9 shows the control operation of the circuit. The bipolar transistor 3 is operated as a switch responsive to the base voltage in the pulse wave applied between the base and emitter by the control unit 3a (FIG. 9 (1)). When the switch closes at time t.sub.1, the collector current, increasing linearly with time flows through the horizontal deflection coil 1. At this point, the tuning capacitor 2 is momentarily charged. Then, when the switch opens at time t.sub.2, the current in the horizontal deflection coil 1 decreases to zero, charging the tuning capacitor (t.sub.3). The tuning capacitor 2 then starts to discharge through the horizontal deflection coil 1. This reverses the current flowing through the horizontal deflection coil (t.sub.3 -t.sub.4). Then, in the LC tuning circuit, which consists of only the horizontal deflection coil 1 and the tuning capacitor 2, charging of the tuning capacitor 2 starts in the reverse direction. However, due to the damper diode 4 operating in parallel with that circuit, the terminal voltage of the horizontal deflection coil 1 becomes higher than voltage E of the power source 6 immediately after the current in the horizontal deflection coil 1 exceeds the maximum value (t.sub.5). This results in the application of forward voltage to the damper diode 4. Current in the horizontal deflection coil 1 flows through the damper diode (the damper current is shown in FIG. 9(4)), charging the power source 6, and the current then gradually decreases. When the current is zero, the switch closes again at time t6 (corresponding to time t.sub.1). By repeating this operation, sawtooth current is conducted through the horizontal deflection coil. FIG. 9(5) shows the voltage generated in the horizontal deflection coil at that time.
FIG. 8 shows the fundamental horizontal deflection circuit. In addition to this circuit, various other types of circuits have been proposed. FIG. 10 shows a circuit intended to produce what is called an S-shaped correction. This circuit corrects excessive elongation at both edges of the television screen. In addition to a horizontal deflection coil 1, a tuning capacitor 2, a switching element 3, and a damper diode 4, this circuit contains a choke coil 11, a correcting capacitor 12, and a horizontal positioning adjustment 13, with a power source 6 located in a different position from that of the circuit in FIG. 8. FIG. 11 shows a circuit which can be used to adjust the amplitude of the horizontal deflection coil current, because it is equipped with an amplitude adjusting coil 14 and other components. However, the principle of control of this circuit is the same as that of the circuit in FIG. 8.
It is therefore an object of the present invention to provide horizontal deflection circuits capable of highfrequency operation without distortion to the horizontal deflection coil voltage.
An additional object is to provide horizontal deflection circuits using conductivity modulation MOS FETs for high frequency operation, for example, with horizontal deflection frequencies of 30-60 kHz.